What's the consideration for this contract?
Speculation: maybe the options they earn when they work there have some provision like this. In return for the NDA the options get extended.
For example, you may join a company and be given options to buy 10,000 shares at $5 each with a 2 year vesting schedule. They may begin vesting immediately, meaning you can buy 1/24th of the total options each month (or 614 shares). Its also common for a delay up front where no options vest until you've been with the company for say 6 or 12 months.
Until an option vests you don't own anything. Once it vests, you still have to buy the shares by exercising the option at the $5 per share price. When you leave, most companies have a deadline on the scale of a few months where you have to either buy all vested shares or forfeit them and lose the stock options.
And there are advantages to exercising: many (most?) companies take back unexercised shares a few weeks/months after you leave, it kicks in a CGT start date, so you can end up paying a lower CGT tax when you eventually sell
You need to understand all this stuff before you make a choice that's right for you